The shortened version of That's what she said (aka TWSS), for use when in a professional setting. The benefit of saying "that's what" is that people who understand TWSS jokes will naturally fill in the implied "she said" whereas others will think nothing of the statement.
Using "that's what" is superior to TWSS for three reasons. It's shorter to blurt out and therefore wittier (the "twiss" pronunciation is hardly understood yet), it's ambiguous and may refer to that's what he said without thinking about it beforehand, and it allows the user to easily brush it off as an accidental exclamation or the beginning of another sentence.
May also occur accidentally at work or school after a weekend bachelor/bachelorette party where "That's what she said" jokes run rampant and the speaker catches themselves before the full phrase comes out.
Boss, in a company meeting: "The product needs to grow bigger and faster to meet our customer's needs"
Employee, back from Vegas: "That's what - " (stops abruptly, friends snicker)
Boss: "What was that?"
Employee: "Oh, er... I was just going to say, 'that's what I was thinking' "
Professor: "Everyone did great on the midterm which concludes the first part of our semester. You're a third in already - and it's not even getting hard yet!"
Student: "That's what..." (students laugh, but the professor ignores it because they generally ignore students anyway)